The present invention relates to a servo apparatus that provides an expanded pull-in capable region to prevent cycle slipping, and it relates particularly to such a circuit which is useful in an optical disk apparatus, PLL circuit or the like.
In a conventional servo apparatus, for example, an optical disk apparatus, as shown in FIG. 2.
The servo apparatus includes: a subtracter 1 for subtracting an output signal of an adder 4, from an input signal so as to supply the result of subtraction to a system 2 to be driven; an adder 6 which is provided to assume that a disturbance is added to the output of the system 2; an encoder 3 for converting the output of the adder 6 into an electric signal; and a differentiation circuit 5 for differentiating the output of the encoder 3, the respective outputs of the encoder 3 and the differentiation circuit 5 being added to each other in the adder 4.
More specifically, a system which corresponds to the system 2, is constituted by a tracking actuator in an optical disk reproducing apparatus, for instance. The tracking actuator 2 is driven in accordance with an input signal so as to move a light beam in a direction perpendicular to the tracks of an optical disk (not shown here). An amount X of movement of the light beam is converted to an electric signal by a pick-up. In this case, however, there may not be a linear relationship between the converted electric signal and the movement amount X, but there may occurs frequent variation therebetween. The variation is approximately expressed as a sine wave (sin X). Accordingly, the pick-up can be regarded as the encoder 3.
The output of the encoder 3 is applied to the differentiation circuit 5 where it is differentiated, and a velocity (cosx) of the light beam is produced from the differentiation circuit 5. The respective outputs of the encoder 3 and the differentiation circuit 5 are added to each other in the adder 4 and the output of the adder 4 is applied to the subtracter 1.
The subtracter 1 subtracts the output of the adder 4 from the input signal and supplies the result of subtraction to the system 2.
Thus, closed loop phase advance compensation is performed by the differentiation circuit 5, which makes it possible to accurately and stably operate a servo system for following the light beam to a destination track.
As described above, in the conventional servo apparatus, a velocity component of the light beam is obtained by differentiating the output of the encoder 3 by means of the differentiation circuit 5. Indeed, the output (cosx) of the differentiation circuit 5 substantially corresponds to the velocity of the output (x) of the system 2, that is, the movement speed of the light beam, in the linear operation area, that is, in a area where the variation X is relatively small, after the servo is pulled in. However, before the servo is pulled in, in a non-linear operation area where the variation X is relatively large, the output (cosx) cannot be a true velocity component (dx/dt) of the output (x) from the system 2, but is instead a signal which is obtained by differentiating the output (sinx) of the encoder 3. As a result, the servo pull-in-capable region is so narrow that cycle slipping is apt to occur.
This will be further described with reference to FIG. 3. In the case where the servo apparatus in FIG. 2 is further subjected to phase place analysis, the output of the encoder 3 is a periodic function with respect to the variation X and the system 2 is a second order system as shown in FIG. 3 In FIG. 3, the abscissa and ordinate express the position x of the light beam and the movement velocity thereof, respectively. In FIG. 3, a group of curves represent loci of the light beam which indicate variation in the position and movement velocity of the light beam in the case where the disturbance x is intentionally applied to cause the position of the light beam and the movement velocity thereof to vary forcibly. The positions which are designated by integers on the abscissa of FIG. 3 represent the track positions. That is, the track positions are designated by -2, -1, 0, 1, 2 and the like. As mentioned above, the ordinate indicates the movement velocity of the light beam. When the movement velocity is of positive, the light beam moves along the locus in a direction where the position number X increases, that is the light beam moves from left to right in FIG. 3. On the other hand, when the movement velocity is of negative, the light beam moves along the locus in a direction where the position number X decreases, that is the light beam moves from the right to left in FIG. 3. For example, assuming that the light beam tracing on the track 0 is caused by a disturbance Xd to forcibly shift to a position A which is defined by a position -1.5 and a velocity 1.0 in FIG. 3, thereafter, the light beam starts moving from the position A as a start position and tracing the locus rightwordly with variation in the movement velocity. However, since the movement velocity is maintained positive even when the light beam arrives at the right-most side of FIG. 3, no light beam is focused on any tracks in FIG. 3. Next, assuming that the light beam is forcibly shifted to a position B due to the disturbance Xd, the light beam traces the locus rightwardly from the position B and then the light beam is focused on the track -2. This results in pulling the light beam in the track -2.
As is apparent from FIG. 3, in order to stop the pickup on a track at the position O, it is preferable to turn a servo loop on at the position -0.25 (the position preceding the position O by the pitch 0.25). The reason for this is that at this position C the velocity becomes equal to about 0.75, and it is only possible to accurately stop the pickup on the track at the position O if the velocity is not more than 0.75.
If the servo loop is turned on at the position (in the left side in FIG. 3) preceding the position -0.25,
cycle slipping is caused even if the velocity is smaller than 0.75, which causes the pickup to move to a track (in the right side in FIG. 3) beyond the position O.
The pull-in-capable region could be expanded if the servo band is expanded, but this would result in the servo band being expanded beyond necessity.